The present invention relates to a white balance adjusting circuit for a projection television system having three CRTs, and more particularly to a white balance adjusting system for automatically controlling balance of color when quantities of light of the three primary colors decrease with time.
In recent years, in order to provide a wider screen for a television system, it has become common to employ a projection television system.
Referring to FIG. 4, a color projection television system has three color picture tubes 1, 2 and 3 each lined with phosphoric material for producing one of the three primary colors, red, green and blue. The color beams are projected on a screen 7 through projection lenses 4, 5 and 6 which are provided in front of the picture tubes 1, 2 and 3 so that a pictorial image appears on the screen 7.
In the system, since each optical axis of the red and green beams are projected on the screen 7 at an incident angle .theta., a keystone distortion which results in a trapezoidal image occurs. In order that the distortion may be so corrected as to adjust convergence of the red, green and blue beams on the screen 7, the red and blue beams emitted from the picture tubes 1 and 3 are distored in a direction opposite to that of the distortion. In addition, a phosphoric screen of the red and blue picture tubes 1 and 3 are slightly tilted to improve focusing.
A rear projection system which is one of the systems of the projection television is shown in FIG. 5. The color picture tubes (CRTs) 1, 2 and 3 for producing red, green and blue colors produces a pictorial image which is expanded by projection lenses 4, 5 and 6. The image is reflected on opposing reflecting mirrors 8 and 9 and reproduced on a light-transmissible screen 7a.
FIG. 6 shows another projection television system where the image is reproduced on a reflecting screen 7b from the front thereof.
There are other types of projection system such as a refraction system shown in FIG. 7 and a reflection system shown in FIG. 8 dependent on the type of color picture tubes 1. In the refraction system, each of the lenses 4, 5 and 6 is provided in front of a fluorescent screen 1a of the respective picture tubes 1, 2 and 3. The lenses 4, 5 and 6 expand and reproduce the image on the reflecting screen 7b.
To the contrary, in the reflection system, each of the picture tubes 1, 2 and 3 has a fluorescent screen 1a and a reflecting mirror 10 provided in the tube. The image reflected on the reflecting mirror 10 is expanded through a Schmidt lens 11 and reproduced on the reflecting screen 7b.
In accordance with the rear projection system shown in FIG. 6, since the picture tubes 1, 2 and 3 and the lenses 4, 5 and 6 can be disposed adjacent the screen 7b, the television system can be reduced in size. The light-transmissible screen 7a is made of a material having a high light transmittance, such as a Fresnel lens and a lenticular sheet.
In the projection color television system, where there are three picture tubes for red (R), green (G) and blue (B), white balance is obtained by adjusting ratio of light between the tubes into red light of 30%, green light of 59%, and blue light of 11% at the production of the receivers. However, the fluorescent screen 1a of the picture tubes has a low luminous efficacy particularly for blue. Accordingly, in order to maintain the balance with the other colors, current supplied to the blue picture tube is increased. Thus the joule heat caused by the current heats the screen of the blue picture tube, thereby quickly deteriorating it.
Hence, as the screen of the blue picture tube deteriorate with time, the luminous efficacy of the blue light is further decreased. As a result, the quantities of red and green lights relatively increases. The picture on the screen 7 will thus gradually assume a yellowish color. The color can be corrected by manipulating a dial for controlling the color balance. However, the adjusting operation is complicated.
In order to solve the problem, there is proposed a white balance control system which is shown in FIG. 9. The control system comprises a video signal output circuit 12, a signal processing circuit 13, a synchronizing circuit 14, and color output control circuits 15, 16 and 17. The signal processing circuit 13 produces red, green and blue video signals in accordance with the output signal of the output circuit 12. The synchronizing circuit 14 controls the projection timing of the video signals applied to the control circuits 15 to 17 to prevent deflection of color and decrease of resolution. The video signals are applied to the R-output control circuit 15, G-output control circuit 16 and B-output control circuit 17, the output signals of which are fed to respective picture tubes 18, 19 and 20 so as to control the light emitting operation thereof. As a result, each of the picture tubes 18, 19 and 20 produces an image which is projected on to a light-transmissible screen 24 through a corresponding one of projection lenses 21, 22 and 23.
Light quantity sensors 25 are provided on the screen 24, for example at the four corners of the screen where the pictorial image is not affected, to detect the quantity of light projected on the light transmissible screen 24.
The output signals of the light quantity sensors 25 are fed to a white balance control circuit 26 where a signal detected by the light quantity sensor 25 is split into red, green and blue components. The ratio of quantities of light corresponding to each components is calculated and compared with a reference light quantity ratio. Intensity of beam of each primary color is adjusted in accordance with the comparison so as to reproduce appropriate white on the screen in a wide range from high luminance to low luminance.
In the system, since the control is based on the signals corresponding to split color components, the accuracy of the adjustment depends largely on the quantity of a light splitting device. Moreover, the light quantity sensors 25 provided on the screen 24 are apt to be affected by outer light coming in from outdoors. More particularly, when the lights have poor luminance, the sensors 25 cannot accurately detect the quantity of light.